Star coupler for an optical communication network

ABSTRACT

An interconnectable star coupler for an optical communication network is formed using 1×2 branching circuits, 2×2 branching circuits, and optical waveguides. Branching circuits may be connected by intersecting optical waveguides. The interconnectable star coupler is built in such a manner that the angle between waveguides meets particular criteria based on the critical angle of the waveguide.

This is a division of application Ser. No. 08/026,054, filed Mar. 4,1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical LAN (local area network) anda protocol used in the optical LAN. More particularly, the inventionrelates to an optical LAN in which a collision on the network isdetected, and the result of collision detection is used forcommunication control.

2. Description of the Related Art

In a general LAN, a plural number of nodes are connected to a bus. Thenodes communicate with one another through the bus. One of the busesused in the LAN is a broadcasting bus. When using the broadcasting bus,a signal sent by a node can be simultaneously received by all of thenodes. ETHERNET (Trade Mark) is well known as one type of LAN using thebroadcasting bus. The protocol used in ETHERNET is called a CSMA/CD(carrier sense multiple access/collision detection) system, prescribedin IEEE 803.3. In ETHERNET, coaxial cables are used for transmissionmedium. The nodes are connected by the coaxial cables. A node which willsend a signal checks whether or not a signal from another node ispresent on the coaxial cable. If it is not present, the sending nodestarts the signal transmission. Actually, there is the possibility thattwo nodes simultaneously send signals. This state is called a collision.In ETHERNET, the collision is detected in the form of a voltage level inthe coaxial cable.

After detecting the collision, the node sends a jamming signal for apreset time, and then is set to a random-time stand-by mode. The jammingsignal must be set to be longer than the maximum Maximum round trip timeof the network, in order to broadcast the collision to all of the nodesconnected to the network. The random-time stand-by mode is provided inorder to avoid such a situation that a plural number of nodes fail tosend, and if those nodes simultaneously start to send signalsimmediately after a communication channel becomes idle, the collisionoccurs again.

The optical communication system has gradually been used also in theLAN. In the LAN using the optical fiber as the transmission media, thenumber of nodes cannot be increased by simply increasing the number oftaps, although it can be increased so in the LAN using the coaxial cableas transmission media.

To solve the problem, there is a proposal of a new optical communicationnetwork in which each node is provided with two separate ports, one fortransmission and the other for reception, and the nodes are coupledthrough a star coupler. For the proposal, reference is made to E. G.RAWSON ET AL., "Fibernet: Multimode Optical Fibers for Local ComputerNetworks", IEEE Transaction on Communications, Vol., COM-26, No. 7, p395(1978).

The optical communication network using the star coupler isschematically illustrated in FIG. 1. In the figure, reference numerals26a and 26b designate optical fibers; 27, nodes; 25, a star coupler ofthe mixing rod type; and 24, terminals. A signal output from each node27 is converted into a light signal by a light emitting element 22 ofthe node. The light signal is supplied through the optical fiber 26aassociated therewith to the star coupler 25. The light signalstransmitted from the nodes are all mixed by the star coupler 25, andthen are distributed to the light sensing elements 23 of the nodes,through the related optical fibers 26b. The light signal is convertedinto an electrical signal by the related light sensing element, andsupplied to the node 27. In the communication network thus arranged, asignal transmitted from one node is transmitted to all of the nodes,viz., the network has a broadcasting function. Accordingly, thecommunication network, which is similar to ETHERNET, can be constructed.

In the proposal, as the number of nodes coupled with the star coupler isincreased, the level of a receiving signal is decreased in each node.One of the possible ways to solve the level down problem is to extendthe network by additionally using star couplers and relay amplifiers.This approach suffers from another problems, however. The star coupler,when receiving a signal from a node, sends it also to the receiving portof the same node. Accordingly, a feedback loop is formed between thestar couplers interconnected. If a relay amplifier is located betweenthe star couplers, an oscillation occurs. When the star coupler is used,the number of nodes that can be connected to the network is limited tothe number of terminals of one star coupler.

As described above, the signal transmitted from a node is distributed tothe receiving port of the same node. This makes it difficult to detectthe collision. That is, since the distribution ratios of actual passivestar couplers are not uniform, it is difficult to apply thelevel-difference detecting method for the collision detection.

To cope with the problem, a CRV (code rule violation) method wasproposed for a collision detecting system, which is to be applied forthe network using the passive star coupler as shown in FIG. 2. The CRVwas discussed by Oguchi et al., in their paper "Study on ArrangingCollision Detecting Circuits for Optical Star Networks", The Institutionof Electronics and Communication Engineers, Optics/Radio Section,National Convention Record 341, 1982.

The CRV method is constructed on the basis of the fact that in theManchester coding system used in ETHERNET, one-bit information isexpressed by two bits, that is, utilizes such redundancy of theManchester coding system.

In the Manchester coding system, the leading edge at the central part ofone period of the reference clock signal shown in FIG. 2(a), is assignedto logical "1" of data, and the trailing edge, to "0" of data. FIG. 2(b)shows an example of the Manchester code, which represents data "110111".As seen from the figure, in the normal state of the Manchester code, theduration of a H (high) level state or a L (low) level state is withinone period of a reference clock signal (FIG. 2(a)).

Let us consider a case where a collision signal as shown in FIG. 2(c)collides with a transmission signal as shown in FIG. 2(b). As the resultof the collision, an intensity distribution of the receiving signaltakes a profile as shown in FIG. 2(d). The receiving signal, whendemodulated, has a bit pattern as shown in FIG. 2(e). The H level stateof which the duration exceeds one period of the reference clock signalis found in the demodulated signal. Thus, when a code (code ruleviolation code), which should not exist, is detected, a CRV signal asshown in FIG. 2(c) is generated. The collision signal shown in FIG. 2(c)represent phase-shifted Manchester codes. Since the nodes are notsynchronized, phases where the Manchester codes are added areindefinite.

The CRV method is based on the rule that when a code (CRV code), whichshould not exist, is detected, it is deemed that a collision occurred.As seen from the rule, the collision is detected on the probabilitybasis, but use of hardware properly selected will suffice for practicaluse.

To solve such a problem that the number of nodes that can be connectedto the network is limited, the applicant of the present PatentApplication proposed a new technique in Published Unexamined JapanesePatent Application No. Hei. 3-296332. In the proposed technique, of aplural number of transmission coefficients, which are for describing thetransfer characteristic of a star coupler, the transfer coefficient ofthe signal transfer between a pair of input and output terminals of anode is set to 0, so that no feedback loop is formed when plural starcouplers are combined. In an optical communication network in which starcouplers are interconnected as described in the specification of theabove publication, a signal that is transmitted from a node will neverreturn to the node per se. Also in the network, the collision detectingmechanism can readily be realized in a manner that a receiving port isconstantly monitored in a transmission mode, and if a signal is detectedat the receiving port, it is determined that a collision has occurred.

Further, in the proposed network, a node can receive a signal fromanother node even if it is sending a signal. Thus, the node canconcurrently perform the transmitting and receiving operations. In otherwords, the optical communication network in the Patent Applicationserves as a bidirectional bus.

A technique that a single optical fiber is used for a bidirectionalcommunication in the network including the combination of star couplers,is also disclosed in the co-pending U.S. patent application Ser. No.07/813,443, filed by the Applicant of the present patent application.The co-pending U.S. patent application Ser. No. 07/873,448, filed by theApplicant of the present patent application, describes that amultichannel LAN can be constructed in which, by multiplexingwavelengths in the network, a plural number of broadcasting buses usinga single optical fiber as a transmission media are arranged in parallel,and that a multimedia LAN which can handle both the data communicationand real-time responsible signals, such as audio and video signals, canbe constructed by using the plural broadcasting buses.

For the operation of the multichannel LAN, the CSMA/CD system istheoretically discussed (by Ikebata and Okada "Multi-Channel CSMA/CDwith Hybrid Load Distribution/Region Distribution Scheme", Trans. ofIECE (in Japanese) (B), Vol., J70-B, No. 12, pp1466-1474 (1987)).

The bidirectional communication system rejects the use of the collisiondetecting method in which the node constantly monitors its receivingport, and when detecting a signal at the receiving port, it decides thatthe collision occurred. In the bidirectional bus, to make abidirectional communication by using the protocol of the linecompetition type, one will probably encounter such a situation that justbefore the communication between first and second nodes will start, athird node starts to send a signal. The collision of the third party issimilar to the collision with another transmission node in theunidirectional communication.

In the communication network, for example, ETHERNET, which usesbroadcasting buses, a signal transmitted by a node can be received byall other nodes. This broadcasting feature is disadvantageous insecuring the secrecy of communication.

To solve the problem, the Applicant of the present patent applicationproposed a novel technique to keep away from the collision with thethird node in patent application Ser. No. Hei. 3-97405. In thetechnique, after sending the signal, a sending node still continues tomonitor the broadcasting bus for a time period τ1, which is longer thana go/return propagation delay τ0 of the broadcasting bus. A respondingnode starts to return a response signal after a time τ2, which is longerthan the time τ1, since the responding node receives a packet destinedthereto.

In a case where the line-competition type protocol is used forcommunication on the communication network of the broadcasting bus, asituation where plural nodes simultaneously send signals through thebroadcasting bus inevitably occurs; that is, the collision inevitablyoccurs. If the node starts the signal transmission after monitoring astate of the broadcasting bus, there is the probability that pluralnodes, not yet knowing the signal transmission of other parties, startto send signals at time intervals each shorter than the Maximum roundtrip time of the broadcasting bus.

In the conventional line-competition type protocol, e.g., the CSMA/CDsystem, the colliding nodes send jamming signals for a preset time(substantially equal to the Maximum round trip time of the broadcastingbus). The nodes are placed to the random-time stand-by mode, and thenstart again the signal transmission. The transmission of the jammingsignal ensures a reliable collision detection. Where the collisionoccurs, no effective communication is performed and only the jammingsignals flow through the broadcasting bus. The send requests issued fromthe colliding nodes are left unremoved. The latent communication demandsare accumulated in the form of the random-time stand-by.

In the present specification, a state that two nodes simultaneously sendsignals within the Maximum round trip time of the broadcasting bus iscalled a 2-node collision. States that three and four nodessimultaneously send signals are called 3-node collision and 4-nodecollision, respectively. The collision of three or more nodes isgenerally called a multiple-collision. Accordingly, the 2-node collisionis not the multiple-collision. Most of the collisions occurring on thebroadcasting bus is the 2-node collision. The multiple-collision rarelyoccurs on the broadcasting bus. This will be described.

It is assumed that sending requests (referred to as calls) are randomlygenerated from a plural number of nodes connected to the broadcastingbus, and that an average frequency of call occurrence per unit time isΛ. When 1000 calls are generated every second, the average frequency Λis 1000 calls/sec. The phenomenon is expressed by the random process,called a Poisson distribution. The Poisson distribution is the functionto provide a probability that n number of calls occur when thebroadcasting bus is monitored for a preset time τ. n generally indicatesa positive integer. The Poisson distribution is given by the followingequation.

    Pτ(n)=e.sup.-Λτ /n| (n≧1)            (1)

When n=0, the Poisson distribution is given by the following equation.

    Pτ(0)=e.sup.-Λτ                             (2)

When Λ=1000 calls/sec. (=10³ calls/sec.), the probability Pτ that onecall is observed during τ=50 μsec is

    Pτ(3)=1.98×10.sup.-5.

The probability Pτ that no call is observed during τ=50 μsec. is

    Pτ(0)=0.951.

Where τ is the Maximum round trip time, if one call is observed duringthe time period τ=50 μsec, then no collision occurs. If two calls areobserved, 2-node collision occurs. If three calls are observed, 3-nodecollisions occurs. Pτ(0) indicates that when the broadcasting bus ismonitored during τ=50 μsec, no call occurs, viz., the line is left idle.

When the probability of 2-node collision occurrence is compared withthat of 3-node collision occurrence, then we have

    Pτ(3)/Pτ(2)=1.60×10.sup.-2 =1.6%,

if Λ=1000 calls/sec and τ=50 μsec. The probability comparison indicatesthat in most cases, 2-node collision occurs, and in rare case, 3-nodecollision occurs. It is known that as the average call-occurrencefrequency Λ per unit time, the percentage of the 3-node collisionsbecomes larger. In the case of large Λ, e.g., Λ=10⁴ calls/sec,Pτ(3)/Pτ(2)=about 20%. In the graph of FIG. 3, the abscissa represents Λ(call/sec) and the ordinate, Pτ(3)/Pτ(2).

The experimental results show that the average call-occurrence frequencyΛ in ETHERNET is at most 30 calls/sec. It is also known that the peakoccurrence of calls is 50 to 60 times as large as the average value perday. For this, reference is made to J. P. Snoch and J. A. Hupp,"Measured performance of an Ethernet Local Computer Network",Communications of A.C., Vol. 23, No. 12, pp711 to 729 (1980).Accordingly, it is seen that λ=1000 calls/sec is approximately theinstantaneous maximum value of the call occurrence frequency.

As seen from the above description, if the 2-node collision can besuppressed, degradation of the channel utilization owing to thecollision can be remarkably reduced. Further, it will be understood thatwhen the 2-node collision occurs, if which of the colliding nodes hasthe priority to use the broadcasting bus can be decided, the result isequivalent to the case of effectively succeeding in avoidance of the2-node collision. In other words, if one of the two colliding nodespermits the other to use the broadcasting bus, the send request of theformer node is canceled. As a result, the accumulation of the potentialsend requests is reduced.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has an object to provide an optical communication network which canefficiently use the broadcasting buses in the network in a manner thatwhen the collision occurs in a broadcasting bus, the type of thecollision is discriminated, and if it is the 2-node collision, thepriority to use the broadcasting bus is determined for the nodescolliding with each other.

To achieve the above object, there is provided an optical communicationnetwork in which a plural number of nodes are connected to abidirectional broadcasting bus, and a node communicates with anotherusing the packets, wherein each node comprises: carrier detecting meansfor detecting a carrier on the broadcasting bus; and jamming detectingmeans for detecting a jamming state of received signals.

In the optical communication network, the carrier detecting meansdetects the 2-node collision, and the jamming detecting means detectsthe multiple-node collision.

A packet generated by each node includes a code train representative ofthe priority level of the packet. When the 2-node collision occurs, thecode trains of the packets of the colliding nodes are compared with eachother. Which of the colliding nodes first gains the right to use thebroadcasting bus is determined on the basis of the comparison result.

In the optical communication network thus arranged, in a case where whena node will send a signal, a carrier is detected on the broadcastingbus, if the node starts to send a signal in disregard of presence of thecarrier, the signal from the node will interfere with a signal sent fromanother node. The jamming state of the received signal indicates thatthe collision of two or more nodes has occurred on the broadcasting bus.Under this condition, if the signal transmission starts, the collisionof three or more nodes will occur. Thus, when a collision occurs in thenetwork, each node can discriminate the type of the collision, the2-node collision or the multiple-node collision, by detecting both thecarrier and the jamming state of the received signal.

The communication system of the invention, when a collision occurs,discriminates the type of collision, the 2-node collision and themultiple-node collision. When the collision is of the 2-node type, whichof the priority levels of the colliding nodes to use the broadcastingbus is higher is determined by using the priority codes previouslyassigned to the packets. With this feature, the communication system canmore efficiently use the communication channels than the conventionalcommunication system of the type in which the nodes randomly competesfor seizing the lines.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrated presently preferred embodimentsof the invention and, together with the general description given aboveand the detailed description of the preferred embodiments given below,serve to explain the objects, advantages and principles of the presentinvention. In the accompanying drawings,

FIG. 1 is a diagram showing a conventional optical LAN using a starcoupler;

FIGS. 2(a) through 2(f) are a timing chart for explaining the principleof the code rule violence method;

FIG. 3 is a graph showing a variation of the ratio of 3-node collisionto 2-node collision with respect to the number of calls per unit time;

FIG. 4 is a diagram schematically showing an optical communicationnetwork according to an embodiment of the present invention;

FIG. 5 is a block diagram showing the construction of a node used in thecommunication network of FIG. 4;

FIG. 6 is a diagram showing the construction of a star coupler with 8terminals, which is used in the communication network of FIG. 4;

FIG. 7 is a block diagram showing the construction of another node usedin an optical communication network according to a second embodiment ofthe present invention,

FIG. 8 is a diagram showing a status transition of a sending node in aprotocol for avoiding the 2-node collision on a network having a singlebidirectional transmission channel;

FIG. 9 is a diagram showing the format of a packet used in the protocolof the invention;

FIG. 10 is a diagram showing a status transition of a sending node in aprotocol for avoiding the 2-node collision on a network having amultiple of bidirectional transmission channels;

FIG. 11 is a diagram showing a status transition of a sending node in aprotocol for avoiding the 2-node collision and for secure communicationon a network having a single bidirectional transmission channel;

FIG. 12 is a diagram showing a status transition of a responding node inthe protocol for avoiding the 2-node collision and for securecommunication on a network having a single bidirectional transmissionchannel;

FIG. 13 is a diagram showing a packet format equivalent to the packetformat of FIG. 9 which additionally uses a sign indicative of a packettype;

FIG. 14 is a diagram showing a status transition of a responding node inthe protocol for avoiding the 2-node collision and for securecommunication, the status transition being featured by provision of apreset waiting time in the response of the node;

FIG. 15 is a diagram showing a status transition of a sending node in aprotocol for avoiding the 2-node collision and for secure communicationon a network having a multiple of bidirectional transmission channels;

FIG. 16 is a diagram showing a first embodiment of a wavelengthmultiplexing transceiver for the wavelength multiplexing in an opticalcommunication network;

FIG. 16(a) is an enlarged diagram showing laser array 106 of FIG. 16.

FIGS. 17(a) and 17(b) are a plan view and a side view showing a secondembodiment of a wavelength multiplexing transceiver for the wavelengthmultiplexing in an optical communication network;

FIG. 17(i c) is an enlarged diagram of the saw-tooth grating of concavegratings 111a and 111b of FIG. 17(a).

FIG. 18 is a diagram showing a third embodiment of a wavelengthmultiplexing transceiver for the wavelength multiplexing in an opticalcommunication network;

FIG. 19 is a schematic diagram showing a perspective view of thewavelength multiplexing transceiver shown in FIG. 18;

FIGS. 20(a) through 20(c) are diagrams showing a fourth embodiment of awavelength multiplexing transceiver according to the present invention;

FIG. 21 is a diagram showing a fifth embodiment of a wavelengthmultiplexing transceiver according to the present invention;

FIG. 22 is a diagram showing a sixth embodiment of a wavelengthmultiplexing transceiver according to the present invention;

FIG. 23 is a diagram showing a seventh embodiment of a wavelengthmultiplexing transceiver according to the present invention;

FIG. 24 is a diagram showing an eighth embodiment of a wavelengthmultiplexing transceiver according to the present invention;

FIG. 25 is a diagram showing a first embodiment of an interconnectable5-port star coupler according to the present invention;

FIG. 25(a) is an enlarged diagram of branching circuit unit 203 of FIG.25.

FIGS. 26(a) to 26(d) are diagrams each showing an example of a opticalcoupler;

FIG. 27 is a diagram showing an incident angle ω of light from onewaveguide to another;

FIG.27(a) is an enlarged diagram of the angles of FIG. 27.

FIG. 28 is a diagram showing an example of an interconnectable starcoupler;

FIG. 29 is a diagram showing an interconnectable star coupler with apair of 9-port groups according to the present invention;

FIGS. 30(a) and 30(b) are diagrams each showing an optical waveguidehaving intersecting portions;

FIG. 31 is a diagram showing a single mode optical waveguide;

FIGS. 32 to 37 are also diagrams each showing a single mode opticalwaveguide;

FIG. 38 is a plan view showing an embodiment of an interconnectable starcoupler according to the present invention;

FIG. 38(a) is an enlarged diagram of the optical couplers of FIG. 38.

FIG. 39 is a plan view showing another embodiment of an interconnectablestar coupler according to the present invention;

FIG. 39(a) is an enlarged diagram of the optical couplers of FIG. 39.

FIG. 39(b) is an enlarged diagram of the reflecting means of FIG. 39.

FIGS. 40(a) to 40(c) are diagrams showing an enlarged Evernescentoptical coupler; and

FIG. 41 is a diagram showing an example of an interconnectable startcoupler with four ports constructed using a 1×3 photocoupler.

FIG. 41(a) is an enlarged diagram of optical coupler 315 of FIG. 41.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An optical communication network according to an embodiment of thepresent invention will be described with reference to FIG. 4.

As shown, a plural number of nodes 1 are interconnected through starcouplers 2. In the communication network of FIG. 4, four star couplers 2each with eight (8) terminals. Those couplers may be connected to oneanother. The star couplers 2 are interconnected through bidirectionaloptical amplifiers 3, thereby forming the network. Each node 1 isconnected through a related optical fiber 4 to the related star coupler2. The packet is used for the communication between the nodes 1. Theoptical amplifier 3 may be a semiconductor laser amplifier.

Two types of nodes 1, which may be used for the communication networkshown in FIG. 4, will be described in detail. The first type of node andthe second type of node are illustrated in FIGS. 5 and 7.

The first type of node shown in FIG. 5 will first be described. As shownin FIG. 5, the node 1 comprises a node body 5 and a communicationinterface 6. The communication interface 6 includes a light sensingelement 9, such as a photo diode, and a light emitting element 8, suchas a laser diode, and a optical coupler 7 for multiplexing anddemultiplexing light signals of different wavelengths. The optical fiber4 derived from the multiplexing/demultiplexing device 7 is connected toone of the terminals 14 of the star coupler 2 shown in FIG. 1. Atransmission port 10 is a hardware for signal transmission, and areception port 11 is a hardware for signal reception. The reception port11 is coupled with a carrier sensor 12 and a code rule violation sensor13. When sensing a carrier and a code rule violation, the sensorstransfer signals representative of the carrier and the code ruleviolation to the node body 5, respectively. Thus, the node body 5 andthe communication interface 6 are interconnected through four ways forsignal flow.

The construction of an interconnectable star coupler with eight (8)terminals, which is used in the communication network of FIG. 4, isillustrated in FIG. 6. The star coupler illustrated may be connected toone or more other star couplers. A substrate 18 is made of glass orpolycarbonate. Disposed on the substrate 18 are optical couplers 15 eachhaving a branching ratio of 7:1, optical couplers 16 each having abranching ratio of 2:1, and a optical coupler 17 having a branchingratio of 1:1. Those devices being connected by optical wave guides,thereby forming an integrated optical circuit. In the integrated opticalcircuit thus formed, desired distribution ratios of light powers areobtained. The multiplexing/demultiplexing device 15 having a branchingratio of 7:1 functions to distribute the light power of 7 to themultiplexing/demultiplexing device 16, and distributes the light powerof 1 to other multiplexing/demultiplexing devices. Themultiplexing/demultiplexing device 16 having a branching ratio of 2:1distributes the light power of 2 to the multiplexing/demultiplexingdevice 17, and the light power of 1 to the multiplexing/demultiplexingdevice 16. Eight number of optical fibers 4 are connected to thesubstrate 18. In FIG. 6, Rc represents the radius of curvature of theoptical waveguide. For more details of the interconnectable starcoupler, reference is made to the specification of the co-pending U.S.patent application Ser. No. 07/813,443, filed by the Applicant of thepresent patent application.

In the communication network thus constructed, when only one node sendsa signal, all other nodes connected to the network can receive thesignal. When two nodes simultaneously send signals, one of the sendingnodes can correctly receive the signal from the other, and vice versa.The other nodes than the two sending nodes can receive the signals as ajamming signal. In other words, the communication network has the securefunction. This arises from the fact that the network is bidirectional.When three or more nodes simultaneously send signals, any node canreceive a jamming signal alone. Thus, in the communication network, thesecret of communication contents can be observed in a state that whentwo nodes send signals, one of them sends a signal to the other and viceversa. The details of this is described in Published Unexamined JapanesePatent Application No. Hei. 3-270432.

The construction of another type of node, that may be used for theoptical communication network of FIG. 4, will be described. When thenodes to be discussed are applied for the FIG. 4 network, an opticalcommunication network of the multi-channel type, which is based on thewavelength multiplexing, can be realized.

As shown, the node includes a node body 5, four communication interfaces6a to 6d, each having the same construction as the communicationinterface 6 in FIG. 2, and a wavelength multiplexer 19. In thecommunication interfaces 6a to 6d, laser diodes as light emittingelements emit light signals of different wavelengths λa to λd. Thewavelength multiplexer 19 multiplexes those light signals of thewavelengths λa to λd.

The bidirectional, optical amplifiers 3 in the optical communicationnetwork shown in FIG. 4, as recalled, are semiconductor laseramplifiers. In each amplifier, the range of wavelengths that can beamplified is broad, 50 to 70 nm. If the wavelengths λa to λd areselected to be different by 10 nm from one another, one semiconductorlaser amplifier is capable of amplifying all of the light signals of thewavelengths λa to λd.

Some specific examples of communication methods, or protocols, accordingto the present invention will be described. FIG. 8 is a diagram showinga status transition of a sending node in a first protocol according tothe invention. The protocol of FIG. 8, applied for a single transmissionchannel (broadcasting bus), is designed to avoid the 2-node collision.Variables in FIG. 8 have the following meanings. A variable CSrepresents presence or absence of a carrier sense. When CS is "1", thecarrier is present. When it is "0", no carrier is present. A variableCRV indicates occurrence of the code rule violation. When CRV is "1",the CRV occurs. When it is "0", no CRV occurs. Pi is a priority level ofthe packet sent by a sending node in the form of a variable. Po is apriority level of the packet sent by a responding node in the form of avariable. Signs ">", "<", and "=" indicate "higher" "lower", and "equal"in the priority level. For example, if Pi>P0, the sending node is higherin priority level than the responding node.

When receiving a send request from the higher layer protocol, the nodesenses a carrier on the transmission channel. When the carrier is sensed(CS=1), the node is placed to the random-time stand-by mode, and thensenses a carrier on the transmission channel since presence of thecarrier indicates that another node uses the transmission channel. Ifthe carrier is not sensed (CS=0), the node starts to send a signal. Whenthe header of the packet has been sent, the node senses the carrieragain while continuing the signal transmission. The packet format isshown in FIG. 9. The carrier sensing operation continues until thesignal transmission has normally ended. If carrier is not sensed beforethe transmission ends (CS=0), the signal transmission has normallyended. If any carrier is sensed before the transmission ends (CS=1), acollision has occurred.

Upon sensing of the carrier (CS=1), the sending node switches thecontents of the sending signal to a jamming signal and fetches thesensed signal. The fetched signal contains the header of a packet thatis sent from another node.

The structure of the packet is illustrated in FIG. 9. As shown, theheader contains a preamble, a code train 20 indicative of priority, anaddress of the called station, and an address of the calling station.The code train 20 indicative of priority will be described later.

The sending node compares the code train indicative of thefetched-packet priority with the code train of the packet that has sentby the sending node per se. If the priority level of the packet sent bythe sending node is higher than that of the fetched packet (Pi>Po), thenode continues to sense the carrier for a fixed time period whilesending the jamming signal. After confirming that the carrier of thecompeted node disappears from the transmission channel (CS=0), the nodesends the packet again. When the any carrier does not appear from thetransmission channel (CS=1), the nodes is placed to a random-timestand-by mode. If the priority level of the competed station is higherthan that of the sending node (Pi<Po), the sending node immediatelystops the sending operation and is placed to the random-time stand-bymode. If the priority levels of both stations or nodes are equal to eachother (Pi=Po), the node is set to the random-time stand-by mode afterthe jamming signal is sent for a fixed time period. In the statustransition diagram of FIG. 8, the carrier sensing is limited within afixed period of time as "Carrier sense (jamming signal) within a fixedtime period". The reason for this follows. When by some error, two nodescollide with each other, one of the nodes decides that the prioritylevel of the node per se is higher than that of the other, and the otheralso makes the same decision. As a result, the competition to seize thechannel continues on the transmission channel in an endless manner. Itis for this reason that the carrier sensing time is limited.

A code system to determine the packet priority in a circulating manneris employed for the code train indicative of packet priority. JANKEN,the game of "scissors, "paper", and "stone", will assist yourunderstanding of the code train. (See Appendix A of the following paper:Yasumoto et al., "PROSPEX: A Graphical LOTOS Simulaor for ProtocolSpecification with N Nodes" IEICE Trans. Commun. Vol. E 75-8, No. 10, pp1015-1023 (1992)) Two (2) bits provide four combinations of "00", "01","10", and "11". Of the four combinations, three combinations "00", "01",and "10" are assigned to "stone", "paper", and "scissors", respectively."01" is prior to "00"; "10", to "01"; and "00", to "10". In this way,the priority level is determined in a circulating manner. In otherwords, the packet priority is relatively determined.

In the embodiment of the invention, 24 number of 2-bits codes (totally48 bits) are arranged. The priority is successively determined bycomparing firstly the first code with the second code, secondly thesecond code with the third code, and so on. The priority is given to thecode having first won. The probability that 24 code trains are all atthe same priority level is (1/3)²⁴ =3.5×10⁻¹². Practically, the event ofthe probability will little occur. The priority determining method cangive impartially the rights to use the channel to the nodes.

JANKEN, which consists of only three combinations, "stone", "scissors",and "paper", may be modified such that "11" is additionally applied asthe fourth hand to JANKEN, and "11" is prior to the remainingcombinations, "00", "01", and "10". From the top of the code trainsindicative of priority, "11" is arranged succeeding to them. The numberof successions of "11" indicates the absolute priority of the packet.The absolute priority is given to the packet under a predetermined rule,in consideration of the nature or the contents of the packet.

In the description thus far made, there is no guarantee of succesfulcommunication that when three or more nodes collid. Accordingly, whenthe multiple-collision occurs, it is necessary to take such a controlprocedure that the random-time stand-by mode follows the transmission ofa jamming signal for a preset time period (as in the normal protocol ofCSMA/CD). The multiple-collision can be detected by sensing occurrenceof the code rule violation.

In the status transition diagram of FIG. 8, when a code rule violationoccurs (CRV=1) in the status of "Sense the carrier while sending asignal", "Make the priority comparison while sending a jamming signal"or "Carrier (jamming signal) sense for a preset time", such a controlprocedure is taken.

The responding node constantly monitors the transmission channel. Whenrecognizing a packet directed to the receiving mode per se, it receivesthe packet and sends it to the higher layer protocol. A statustransition of the responding node in the first protocol is simple, andhence the status transition diagram is omitted here.

Another communication method, or a second protocol, of the inventionwill be described. A status transition of a transmitting node in thesecond protocol is illustrated in FIG. 10. The second protocol isapplied to a communication network of which the nodes are of the typeshown in FIG. 7, and parallel transmission channels are arranged inparallel on the basis of the wavelength multiplexing. The statustransition diagram of FIG. 10 is different from that of FIG. 8 in that astatus "Random channel select" is additionally used.

When a send request is received from the higher layer protocol, thesending node simultaneously senses the carriers of the plural channels,and randomly selects one of idle channels and starts to send a signalthrough the selected idle channel. The control procedure of the secondprotocol is substantially equal to that shown in FIG. 8 except theprocessing to be performed when the node is inferior as the result ofpriority comparison. In the status transition of FIG. 8, the sendingnode immediately stops the transmission and is set to the random-timestand-by mode. In the status transition of FIG. 10, immediately afterthe transmission is stopped, the sending node simultaneously senses thecarriers of the channels. If there is an idle channel, the sending nodeselects it and sends a signal through the selected channel. If there areplural idle channels, the node randomly selects one of the idlechannels, and uses it for signal transmission. With provision of thecontrol procedure, if there is an idle channel, the node who wasdefeated in the line competition can send a signal through the idlechannel without idle waiting. Accordingly, the delay time required forthe communication can be reduced. In FIG. 10, the carrier sense isexpressed by a variable of CSn where n indicates the transfer channelnumber. In the status of "Carrier sense for a preset time", if CSn=1,the procedure returns to the carrier sense. This is done in order toavoid such a situation that by some error, the line competitionendlessly continues. In the instant protocol, the network to which theprotocol is applied contains a plural number of channels. Accordingly,the node operation does not enter the random-time stand-by mode, butsearches for an idle channel or channels.

In the second protocol, the responding node sometimes receives a pluralnumber of packets in a simultaneous way. Therefore, the through-put mustbe correspondingly increased. Except this, the control procedure of theprotocol of the responding node is substantially the same as that in thefirst protocol.

In the communication network, the wavelength multiplexing technique isused for forming the multi-channel arrangement consisting of a multipleof bidirectional transmission channels. The technique disclosed in theco-pending U.S. patent application Ser. No. 07/946,192 may be used forthe same purpose, in place of the wavelength multiplexing. It is evidentthat the second protocol may be applied for the communication networkdescribed in the specification of the above patent application.

A third protocol of the invention will be described. The third protocolis equivalent to the first protocol which additionally uses a controlprotocol for secure communication. FIG. 11 is a diagram showing a statustransition of the sending node in the third protocol. FIG. 12 is adiagram showing a status transition of the responding node in the thirdprotocol. The structure of a packet in the third protocol is somewhatdifferent from the packet structure (FIG. 9) in the first and secondprotocols. As shown in FIG. 13, a sign 21 representative of the type ofa packet is additionally contained in the header of the packet. The sign21 consists of two bits. The code train 20 indicative of priorityconsists of 46 bits (23 number of 2-bits pairs). The packet type PT is:"00" for the normal packet, "01" for the packet of the sending node inthe secure communication, and "10" for a response packet of theresponding side in the secure communication. "11" is not defined. InFIGS. 11 and 12 showing status transition diagrams, PT is a variablerepresenting the packet type. R is a variable which is set to "1" whenthe address of the sending node (or sender), from which the receivedpacket originates, is coincident with the address of the responding nodeor responder. When the addresses of the sender and responder are notcoincident, the variable R is set to "0". A is a variable which is setto "1" when the address of the node, to which the received packet isdestined, is coincident with the address of the sending node, and to "0"when the addresses are not coincident with each other.

The status transition diagram shown in FIG. 11 is based on that of FIG.8. The control procedure for the normal packet after transmission of theheader has been completed is different from that for the securecommunication. The control procedure for the normal packet (PT=00)resembles the procedure as described in the status transition diagram ofFIG. 8. In the control procedure for the secure communication (PT=01),the sending node sends a jamming signal immediately after the completionof the header transmission. In this mode, the transmission is permittedonly when the address of the sender is coincident with that of theresponder (R=1) and PT (packet type)=10. When the address of the sender,from which the packet is received, is not coincident with the address ofthe responder (R=0), the random-time stand-by mode is set up after thejamming signal is sent for a preset time. Also when CRV=1 in theprocessings of "Sense a carrier while sending a jamming signal" and"Sending" (FIG. 11), the random-time stand-by mode is set up after thejamming signal is sent for a preset time.

The responding node or responder operates for procedural control asdescribed in a status transition diagram shown in FIG. 12. As shown, theresponder monitors the transmission channel, receives a carrier (CS=1),and detects the address of the node, to which the received packet isdestined. When the detected address is not coincident with the addressof the responder (A=0), the responder returns to the channel monitoringjob. When those addresses are coincident (A=1) with each other, itdetects the type of the return request, or the packet type. When thepacket is of the normal type (PT=00), it fetches the packets so long asthe carrier is sensed (CS=1), and it transmits the packets to the upperlayer protocol and ends the receiving protocol procedure.

When the addresses of the sender and the responder are coincident witheach other (A=1) and PT (packet type)=1 (secure communication mode), thenode starts the transmission of the jamming signal, and fetches thepacket while transmitting the jamming signal, and transfers the packetto the upper layer protocol. When the code rule violation occurs (CRV=1)during the transmission of the jamming signal, it immediately stops thetransmission of the jamming signal.

According to the secure communication protocol, since the node withwhich the sending node communicates supplies the jamming signal, the2-node collision is intentionally caused to occur on the transmissionchannel. As a result, secrecy of the communication between the two nodescan be protected against the other nodes than the above two nodes.

The fourth protocol of the invention is a modification of the thirdprotocol. In the third protocol, the 2-node collision is forcibly causedto occur. This leads to increase of a probability of occurrence of3-node collision. The increased probability of the 3-node collisionoccurrence impairs the advantageous function of the 2-node collisionavoiding mechanism. In the fourth protocol, the responding node startsto send a signal after time approximate to the go/return delay timeelapses, viz., it does not return a signal immediately after the packetis received. With additional use of the procedure, the probability ofthe 3-node collision occurrence can be kept low and the advantageousfunction of the two-node collision avoiding mechanism can be kept as itis. A status transition diagram of the responding node in the fourthprotocol is shown in FIG. 14. A status transition diagram for the senderin the protocol is the same as that of FIG. 11.

The status transition diagram of FIG. 13, which is based on that of FIG.12, is different from the latter in that a job of "Wait for a presettime" is additionally used located between jobs "Return request sense"and "Receive a signal while transmitting a jamming signal".

The fifth protocol of the invention is equivalent to the second protocolfor multichannel which additionally uses a control procedure for securecommunication in the third protocol. In other words, the fifth protocolis provided with all of functions thus far referred to. FIG. 15 is astatus transition diagram for the sender in the protocol having both thefunction of 2-node collision avoidance and the function of securecommunication. The status transition diagram of FIG. 15 is thecombination of the FIGS. 10 and 11 status transition diagramsrespectively for the second and third protocols. Accordingly, no furtherdescription of the FIG. 15 status transition diagram will be given here.The control procedure in the status transition diagram of FIG. 12 or 14is available for the control procedure of the responder in the fifthprotocol.

As seen from the foregoing description, in the optical communicationnetwork, the type of collision, 2-node collision or multiple-nodecollision, is discriminated. The control procedure for the 2-nodecollision or for the multiple-node collision is selectively usedaccording to the result of discrimination. As a result, the 2-nodecollision can effectively be avoided, the communication channels can beefficiently used, and the delay times required for the packettransmission can be reduced in average.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from-practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and its practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto, and their equivalents.

A first embodiment of a wavelength multiplexing transceiver for thewavelength multiplexing in an optical communication network describedabove is shown in FIG. 16. The first embodiment is a wavelengthmultiplexing transceiver for optical communication in which fourwavelengths are multiplexed. The wavelength multiplexing transceiver iscomprised of an integrated optical circuit substrate 101 containingintegrated optical waveguides, a diffraction grating substrate 104located on the bottom side (viewed in the drawing) of the integratedoptical circuit substrate 101, a photo diode array 105 as a lightsensing array, a semiconductor laser array 106, and an optical fiber 110for input/output for light signals. The photo diode array 105, thesemiconductor laser array 106, and the optical fiber 110 are disposed onthe top side (viewed in the drawing) of the integrated optical circuitsubstrate 101.

The integrated optical circuit substrate 101 is a glass substrate inthis embodiment. A first slab waveguide 102 for transmission, a secondslab waveguide 103, a first optical coupler 107, a second opticalcoupler 108, and wiring optical waveguides 109a to 109k are formed onthe integrated optical circuit substrate 101 by metal ion diffusionprocess. For the waveguide formation by the metal ion diffusion process,reference is made to E. Okuda, I. Tanaka, and T. Yamasaki: "Planargradient--index glass waveguide and its applications to a 4-portbranched circuit and star coupler", Appl. Opt. 23, p1745 (1984). Thewiring optical waveguides 109a to 109k have each 10 μm in diameter, andits waveguide mode is a single mode. The first slab waveguides 102 and103 are each 10 μm thick. The size of the integrated optical circuitsubstrate 101 is: L1=50 mm and L2=40 mm.

A couple of Fresnel reflecting mirrors 104a and 104b as spectroscopingmeans are formed on the diffraction grating substrate 104, which islocated on the bottom side of the integrated optical circuit substrate101. The Fresnel reflecting mirrors 104a and 104b are disposedcorresponding to the first slab waveguide 102 and second slab waveguide103, respectively. The first slab waveguide 102 and the Fresnelreflecting mirror 104a make up a first wavelength multiplexer of theslab waveguide type. The second slab waveguide 103 and the Fresnelreflecting mirror 104b make up a second wavelength multiplexer of theslab waveguide type. The construction of the first wavelengthmultiplexer is preferably the same as that of the second wavelengthmultiplexer. Those multiplexers have not always the same shape, but itis only needed that both the multiplexers have the same structure asviewed in the cross sectional direction.

The semiconductor laser array 106, provided on the bottom side of theintegrated optical circuit substrate 101, is disposed at the locationcloser to the first slab waveguide 102. In the instant embodiment, sincefour wavelengths are multiplexed, five semiconductor laser elements 106ato 106e are contained in the laser array 106. Those laser elements 106ato 106e are arrayed at pitches of 100 μm. The number of required laserelements is the number of multiplexed wavelengths+1. The laser element106e of those elements, which is located closest to the second slabwaveguide 103, serves as a common semiconductor laser element. One ofthe major sides of the laser array 106 is covered with a anti-reflectioncoat. When mounted, the anti-reflection coat side of the laser array 106faces the integrated optical circuit substrate 101.

The first slab waveguide 102 and the laser array 106 with theanti-reflection coat are interconnected with a group of wiring opticalwaveguides 109a to 109e, of which end face pitch is 100 μm. The firstslab waveguide 102 and the Fresnel reflecting mirror 104a cooperate toform a polychrometer. The four wiring optical waveguides 109a to 109d,arrayed at equal pitches, are connected to the first slab waveguide 102,thereby to form a polychrometer output 114.

A common output 113 is disposed at the location substantially inopposition to the polychrometer output 114 on the top side of the firstslab waveguide 102. The common output 113 is connected to the firstoptical coupler 107. A first branch of the first optical coupler 107 isconnected through the wiring optical waveguide 109e to the common laserelement 106e of the laser array 106. A second branch of the firstoptical coupler 107 is connected through the wiring optical waveguide109f to the second optical coupler 108. The common terminal of thesecond optical coupler 108 is connected to the optical fiber 110.

The focal distance of the Fresnel reflecting mirror 104a is 15 mm. Adistance A between the center of the mirror 104a and the image formingplane (polychrometer output 114) is 30 mm. A dispersion at the imageforming plane (polychrometer output 114) is 100 nm (with respect to thecenter wavelength of 800 nm) per 1 mm. The wiring optical waveguides109a to 109d, arrayed at 100 μm pitches, are connected to thepolychrometer output 114 of the first slab waveguide 102. Accordingly, alaser oscillation occurs where wavelengths are multiplexed at 10 nmpitches. In this instance of the embodiment, the array pitch of thelaser array 106 is coincident with the wiring optical waveguide pitch ofthe polychrometer output 114. If required, those are not coincident witheach other.

The laser array 106, the first slab waveguide 102, and the Fresnelreflecting mirror 104a make up a wavelength multiplexing resonanceoptical system, which generates wavelength-multiplexed laser light. Inthe optical system, the common laser element 106e is coupled, withdifference wavelengths, with the remaining laser elements 106a to 106dthrough the Fresnel reflecting mirror 104a, so that laser oscillationoccurs at the respective wavelengths. In more particular, the commonlaser element 106e and the laser element 106a are coupled with eachother, with a wavelength λ1, through an optical path including thewiring optical waveguide 109e, the first slab waveguide 102, the Fresnelreflecting mirror 104a, the first slab waveguide 102, and the wiringoptical waveguide 109a. The common laser element 106e and the laserelement 106b are coupled with each other, with a wavelength λ2, throughan optical path including the wiring optical waveguide 109e, the firstslab waveguide 102, the Fresnel reflecting mirror 104a, the first slabwaveguide 102, and the wiring optical waveguide 109b. The wavelength isdetermined depending on the positional relationship of the common laserelement 6e and the remaining laser elements 106a to 106d to the Fresnelreflecting mirror 104a. For the details of this, reference is made toJapanese Patent Application No. Hei. 3-251677.

The laser light generated at the multiplexed wavelength is output fromthe common output 113. The laser light output from the common output 113of the first slab waveguide 102 is branched by the first optical coupler107. Part of the laser light is output through the second opticalcoupler 108 to the optical fiber 10.

The second slab waveguide 103 and the photo diode array 105 areconnected to each other by the wiring optical waveguides 109h to 109k.The second slab waveguide 103 and the Fresnel reflecting mirror 104bmake up a polychrometer. Four photo diodes (not shown), corresponding tothe number of wavelengths are arrayed in the photo diode array 105. Thearray pitch of the photo diode array 105 is 100 μm. A common input 115of the second slab waveguide 103 and the second optical coupler 108 areconnected by the wiring optical waveguide 109g. The wiring opticalwaveguides 109h to 109k are connected to the output 116 of thepolychrometer of the second slab waveguide 103.

The focal distance of the Fresnel reflecting mirror 104b is 15 mm. Adistance A between the center of the mirror 104b and the image formingplane (polychrometer output 116) is 30 mm. A dispersion at the imageforming plane (polychrometer output 116) is 100 nm (with respect to thecenter wavelength of 800 nm) per 1 mm. The wiring optical waveguides109h to 109k, arrayed at 100 μm pitches, are connected to thepolychrometer output 116 of the second slab waveguide 103.

A light signal coming in through the optical fiber 110 from external isbranched by the second optical coupler 108. Part of the light signal issplit and imaged by the Fresnel reflecting mirror 104a. The split lightsignals of different wavelengths pass through the wiring opticalwaveguides 109h to 109k and reach the photo diodes (not shown) of thephoto diode array 105, where those signals are converted into electricalsignals.

In the embodiment as mentioned above, the modulator/solid laser arraycombination may be used in place of the laser array 106. Further, asolid optical laser amplifier formed by doping the glass substrate withrare earth element may be used. A proper relay optical system may beinserted between the photo diode array 105 and/or laser array 106 andthe integrated optical circuit substrate 101. The photo diode array 105may be replaced with another suitable light sensing element capable ofconverting light signals into electrical signals.

As described above, the spectroscoping systems for transmission andreception have the same structures as viewed in the cross sectionaldirection of the integrated optical circuit substrate 101. Therefore,when manufacturing the wavelength multiplexing transceiver, thetransmitting and receiving portions can be manufactured in the samemanufacturing process. In this respect, the manufacturing process issimplified.

A second embodiment of the invention is illustrated in FIG. 17. FIGS.17(a) and 17(b) are a plan view and a side view showing the secondembodiment. The difference of the second embodiment is different fromthe first embodiment of FIG. 16 in that concave gratings 111a and 111bare used in place of the Fresnel reflecting mirrors 104a and 104b. Theconcave gratings 111a and 111b are formed in a manner that theintegrated optical circuit substrate 101 made of glass is dry-etched toshape a saw-tooth grating 112 in a concave. Accordingly, in the secondembodiment, the number of required components is reduced by one whencomparing with the first embodiment. In the concave gratings, since 500gratings/mm, the grating pitch is 2 μm. The radius of curvature (Rowlandradius) R of the concave grating is 15 mm. A refractive index of theintegrated optical circuit substrate 101 is approximately 1.5. Adispersion on the image forming plane (polychrometer output 114 or thepolychrometer output 116) is 100 nm (with respect to the centerwavelength of 800 nm) per 1 mm.

A third embodiment of the invention is illustrated in FIGS. 18 and 19.In the third embodiment, the invention is applied to a wavelengthmultiplexing transceiver for optical communication of which thewaveguide mode is a multimode.

Since the core diameter of the multimode optical fiber is 50 μm, theoptical waveguide of the wavelength multiplexing transceiver must havethe size corresponding to it. The stripe width of the semiconductorlaser is 5 μm at most, and the thickness of the semiconductor laser,which includes the clad layer, is only 2 to 5 μm. For this reason, it isdifficult to couple the optical waveguide of about 50 μm in diameterwith the semiconductor laser. Laser light emitted from the semiconductorlaser device can be input to the optical waveguide of about 50 μm indiameter without any problem. However, it is very difficult to input thelight emanating from the optical waveguide to the semiconductor laserdevice.

To cope with the above problem, in the third embodiment shown in FIGS.18 and 19, the substrate 101 has a structure consisting of two plasticlayers layered one over the other. The wavelength multiplexingtransceiver when seen in the direction of an arrow B in FIG. 18 isperspectively illustrated in FIG. 19. For a better illustration of thestructure of the substrate 101, the photo diode array 105, thesemiconductor laser array 106 and the optical fiber 110 are contoured bydotted lines. As shown, the substrate 101 is structured such that a thinplastic thin film 117 is layered on a thick plastic thin film 118. Anoptical waveguide circuit including the optical waveguides 109a to 109f,which connects to the laser array 106, is formed in the thin plasticthin film 117. The optical waveguides 109h to 109k connecting to thephoto diode array 105 and the optical waveguide 109g connecting to theoptical fiber 110 are formed in the thick plastic thin film 118. Thethickness d1 of the thin plastic thin film 117 is 10 μm, and thethickness d2 of the thick plastic thin film 118 is 30 μm. The secondoptical coupler 108 is formed by laying the optical waveguide 109f ofthe thin plastic thin film 117 on the optical waveguide 109g of thethick plastic thin film 118. The optical waveguide 109f, which couplesthe second optical coupler 108 with the first slab waveguide 102, isshaped such that a portion 109f of the waveguide closer to the secondoptical coupler 108 is broad, 40 μm, and a portion 109f₂ closer to thefirst slab waveguide 102 is narrow, 10 μm. Accordingly, the opticalwaveguide at the location coupled with the optical fiber 110 is shapedin square, 40 μm×40 μm. The known selective photopolymerization is usedfor forming the optical waveguides in the plastic thin films 117 and118. The selective photopolymerization is discussed by T. Kurosawa, N.Takato, S. Okikawa and T. Okada in their paper "Fiber optic sheetformation by selective photopolymerization, Appl. Opt. 17, p646 (1978).The substrate 101 was formed by laminating two plastic thin films havingoptical waveguides already formed therein. The thin films may be anotherother material than plastic, if it allows optical waveguides to beformed therein.

FIG. 20 shows a fourth embodiment of the present invention. In thefourth embodiment, a wavelength multiplexing transceiver for opticalcommunication is formed as an integrated optical circuit on asemiconductor substrate 131. FIG. 20(a) is a plan view of the wavelengthmultiplexing transceiver; FIG. 20(b) is a cross sectional view takenalong a line of X--X in FIG. 20(a); and FIG. 20(c) is a side view of thesame. The wavelength multiplexing transceivers of the fourth embodiment,and fifth to seventh embodiments to be given later are three-wavelengthsmultiplexing transceivers.

In FIG. 20, reference numerals 134a to 134c, and 135 designatesemiconductor laser elements. The semiconductor laser elements 134a to134c correspond to the semiconductor laser elements 106a to 106c. Thesemiconductor laser element 135 corresponds to the semiconductor laserelement 106e. Photo diodes 136a to 136c correspond to the photo diodes(not shown) of the photo diode array 105 shown in FIG. 16. The structureof the photo diodes 136a to 136c is substantially the same as that ofthe semiconductor laser elements 134a to 134c. When it is fed withcurrent, it serves as a laser diode. When it receives light, itgenerates photo current. The laser elements 134a to 134c are differentin element length from the photo diodes 136a to 136c. The length α ofthe laser elements 134a to 134c is 250 μm, and the length β is 10 μm(the illustration of FIG. 20 roughly shows a layout of elements, and thelayout is not exact in the reduced scale). Such a figure of the photodiode length is selected because the photo diode of 10 μm long cansatisfactorily absorb light. If the element length is selected to belong, the stray capacitance of the element is increased, and cannothandle the received light signals, which are modulated at high speed.The pitch of the laser elements 134a to 134c is 10 μm, and the pitch ofthe photo diodes 136a to 136c is also 10 μm. The width S of the opticalwaveguide is 3 μm. The substrate 31 is: L3×L4=10 mm×10 mm.

The first slab waveguide 102, the second slab waveguide 103, the firstoptical coupler 107, the second optical coupler 108, and the opticalwaveguides for wiring are semiconductor optical waveguides. The portionof the substrate connecting to the optical fiber 110 and the end facethereof on which the photo diodes 136a to 136c are covered with aanti-reflection coat 137. The anti-reflection coat 137 is not formed onthe portion of the substrate where the laser elements 134a to 134c and135. The substrate 131 and the optical fiber 110 are optically coupledwith each other by a coupling lens 138.

The first and second slab waveguide 102 and 103 are provided with theconcave gratings 111a and 111b, respectively. In the concave gratings,since 500 gratings/mm, the grating pitch p is 2 μm. The radius ofcurvature (Rowland radius) R of the concave grating is 15 mm. Arefractive index of the GaAlAs alloy is approximately 3.5. A wavelengthdispersion on the focal plane of the concave grating is 1000 nm per 1mm. Accordingly, the array of 10 μm can multiplex wavelengths at thepitches of 10 nm.

A method of manufacturing the wavelength multiplexing transceiver of thefourth embodiment will be described in brief. The substrate 131 was aGaAs substrate. AlGaAs double hetero structure is epitaxial grown on thesubstrate 131 by a MOCVD (metal organic chemical vapor deposition)method. Si is diffused into other portions than those portions where thelaser elements 134a to 134c and 135, and the photo diodes 136a to 136care formed, so that the double hetero structure is disordered. Thisprocess is called IID (impurity induced disordering) process. Thestructure is dry etched to form the concave gratings 111a and 111b. Inthe fourth embodiment, when the concave gratings 111a and 111b areformed, the respective optical waveguides (slab waveguides and wiringoptical waveguides) are also formed as ridge type waveguides. Followingthe electrode formation and cleavage, the anti-reflection coat 137 isformed on the end face of the substrate 131 by deposition process. Atthis time, the portions of the substrate corresponding to the laserelements 134a to 134c and 135 are covered with a mask.

FIG. 21 is a fifth embodiment of the invention. The fifth embodiment isa modification of the fourth embodiment. In the structure of the fourthembodiment shown in FIG. 20, the mask is required when theanti-reflection coat 137 is formed. This makes it difficult tomanufacture. To cope with this, in the fifth embodiment of FIG. 21, theconcave grating 111a for transmission and the concave grating 111b forreception are opposite to each other in the direction. Further, thelaser elements 134a to 134c and 135 are formed in the end face of thesubstrate 131 different from that where the photo diodes 136a to 136care formed. Since the fifth embodiment is thus constructed, there is noneed of using the mask when the anti-reflection coat 137 is formed onthe end face where the photo diodes 136a to 136c are formed.

FIG. 22 shows a sixth embodiment of the invention. In this embodiment,an optical amplifier 139, such as a semiconductor laser amplifier, isformed in the optical waveguide, which connects the second opticalcoupler 108 to the second slab waveguide 103. The structure of theoptical amplifier 139 is substantially the same as that of the laserelements 134a to 134c and 135. The element length γ of the opticalamplifier 139 is 500 μm, in this embodiment. The optical amplifier 139,which amplifies the received light signal, compensates for the loss ofthe light signal when it passes through the waveguide. The loss by thesemiconductor waveguide is approximately 10 dB/cm, and this figure ismuch larger than 0.1 dB/cm of the glass waveguide. Particularly, in acase where the wavelength multiplexing transceiver is integrated on thesemiconductor substrate, the semiconductor laser amplifier cancompensate for the waveguide loss, which results from the use of thesemiconductor slab waveguides. In this respect, the sixth embodiment canprevent the performance deterioration of the wavelength multiplexingtransceiver when it is integrated.

FIG. 23 shows a seventh embodiment of the present invention. The seventhembodiment is a modification of the sixth embodiment of FIG. 22. Opticalamplifiers 139a to 139c are provided in the waveguide connecting thesecond slab waveguide 103 to the photo diodes 136a to 136c.

Use of the optical amplifiers is effective particularly when thewavelength multiplexing transceiver is integrated on the semiconductorsubstrate. The optical amplifier may be applied for the case using theglass or plastic substrate. The optical amplifier may be any otheramplifier than the semiconductor laser-amplifier. The optical amplifiermay be realized by forming an optical waveguide on the glass substratedoped with rare earth element.

FIG. 24 shows an eighth embodiment of the invention. The eighthembodiment, like the first to third embodiments, is of thefour-wavelengths multiplexing type. Like reference numerals are used fordesignating like or equivalent portions in the first to thirdembodiments. In the eighth embodiment, the substrate 101a is shaped likea trapezoid as viewed in cross section. The upper end faces of the slabwaveguides 102 and 103 are exposed at the stepped portion of thesubstrate 101a. The narrow part of the substrate 101a, which is locatedcloser to the slab waveguide 102, is inwardly cut away to form acut-away portion 101b. A semiconductor laser array 106 is disposed inthe cut-away portion 101b. The laser array 106 contains fivesemiconductor laser elements 106a to 106e. Both end faces of the laserelement 106e is covered with anti-reflection coats. In the case of theremaining laser elements 106a to 106d, only the end faces thereof closerto the first slab waveguide 102 are covered with the anti-reflectioncoat. The laser element 106e of the laser array 106 is connected to thesecond optical coupler 108 by means of the wiring optical waveguide109f. The wavelength multiplexing operation in the eighth embodiment issimilar to that of each of the embodiments as mentioned above. In thepresent embodiment, the light signal is output from the end of the laserelement 106e of the laser array 106, which is far from the first slabwaveguide 102, while in the above-mentioned embodiments, it is outputfrom the first slab waveguide 102.

A photo diode array 105 is disposed in the stepped portion of thesubstrate, which is closer to the second slab waveguide 10. The eighthembodiment is different from the above-mentioned embodiment in that thesecond slab waveguide 103 and the photodiode array 105 are coupled witheach other not using the waveguide. However, the basic spectroscopingand image forming operations are not unchanged.

Also in the eighth embodiment, the optical systems having the samestructure may be used for both the transmitting and receiving sectionsof the wavelength multiplexing transceiver. In this respect, the devicestructure is simplified.

In the embodiments thus far described, the Fresnel reflecting mirrorsand the concave gratings are used for the spectroscoping means in thewavelength multiplexer of the slab waveguide type. Those elements may bereplaced by chirped gratings or a spectroscope of the array waveguidetype.

As seen from the foregoing description, the same structure of thespectoscoping system is used for the transmitting and receiving sectionsof the wavelength multiplexing transceiver, so that the device structureis simplified. Further, the wavelength multiplexing transceiver isformed on the semiconductor substrate in an integrating manner. Themanufacturing process is also simplified. Incorporating the opticalamplifier into the receiving circuit of the integrated optical circuit,which forms the wavelength multiplexing transceiver, can compensate forthe waveguide loss by the semiconductor optical waveguide, eliminatingthe performance deterioration, which results from the integratedfabrication of the wavelength multiplexing transceiver.

FIG. 25 shows a first embodiment of an interconnectable 5-port starcoupler according to a preferred embodiment of the present invention.Three 1×2 equal branching circuits 205 are combined in a tree fashion,thereby forming an equal branching circuit unit 203 with four ports. Asshown, five light-equal branching circuit units 203 are arrayed on asubstrate 201 in a star fashion, thereby forming the star coupler. Alight signal emanating from an optical fiber 202 is equally divided intofour light signals, by the branching circuit unit 203. Those dividedlight signals are distributed to the remaining optical fibers 204,through optical waveguides 201a formed on the same plane of thesubstrate 201. The integrated optical circuit contains five intersectingportions 204 where the optical waveguides 201a intersect.

In the 1×2 equal branching circuit 205, as shown in FIG. 26(a), a lightsignal enters an optical waveguide 231, passes through a mixing part236, and is branched into two optical waveguides 233 and 234. The equalbranching circuit 205 is frequently called a Y branching circuit sinceit is shaped like letter Y. In the case of the equal branching circuit205, two optical waveguides 233 and 234 are directly coupled together.Because of this, its junction loss is small. However, in some lightpropagation modes, its branching ratio is often limited. Specifically,in a single mode, it can branch the light signal at only 1:1 of thebranching ratio, basically.

A 1×2 Evanescent optical coupler (in other words, an optical couplerbased on coupled mode theory), as shown in FIG. 26(b), includes aportion where two optical waveguides 235a and 235b are closely located.This optical coupler functions to transfer a light signal from onewaveguide to another via by the Evanescent coupling. Structurally, anextremely thin medium (clad) of low refractive index is located betweentwo optical waveguides (core), made of medium of high refractive index.Energy is transferred from one waveguide to another via Evanescent wave.In other words, this optical coupler is based on coupled mode theory.This optical coupler is advantageous in that it can take a desiredbranching ratio, but is disadvantageous in that the junction loss islarge as described above.

A light signal coming in through a port of the star coupler shown inFIG. 25 is branched into four light signals by the branching circuitunit 203, and distributed to other ports than the light signal receivingport. Since light has a good rectilinear propagation, the light signalpropagating through one waveguide is little leaked to another waveguideat the intersecting portion 204.

At the intersecting portion 204, no interference occurs between thewaveguides if an angle δ between the waveguides exceeds a value twotimes as large as a critical angle θ of the waveguide. The reason forthis follows. As shown in FIG. 27, an incident angle ω of light from onewaveguide to another is given by the follow equation:

    ω=δ-θ                                    (3)

If ω>θ, no light is induced into another waveguide. Then, we have

    ω>2θ                                           (4)

The critical angle of the waveguide is a maximum angle which allowstotal reflection to occur on the interface between the core and clad ofthe optical waveguide. It is given by

    θ=90 sin.sup.-1 (n2/n1)                              (5)

where n1 is refractive index of the core, and n2 is refractive index ofthe clad (n1>n2). Where the refractive index of the core and clad is 2%,θ is approximately 3° C. Accordingly, in this case, δ must be largerthan 6°.

Where the intersection angle δ is large, the light which does not couplethe waveguides at the intersecting portion 204 increases relative to theother light. This results in increase of loss (transmission loss). Thefact that the transmission loss abruptly increases when the intersectionangle δ decreases below 20°, has been numerically calculated (seeTakahashi and Inagaki "Analysis of the transmission loss in matrixoptical waveguide", The 1992 IEICE (institute ofelectronics/information/communication Engineers) spring conferencerecord, C-192 (1992)). For this reason, δ is preferably lager than 20°.

In the embodiment of FIG. 25, the substrate 201 is made of glass, andthe optical waveguides 201a is a single mode optical waveguide formed byan ion exchange method. It is evident that the material and themanufacturing method are not limited to the just mentioned ones. In thepresent invention, the light signal is branched at 1:1 by the equalbranching circuit 205 of the branching circuit unit 203. Therefore, whenthe invention is applied for the single mode, interconnectable starcoupler, it effectively operates. As a matter of course, the inventionis applicable for the multi-mode, interconnectable star coupler. Thedifference between the refractive index values of the core and clad maybe properly selected.

Interconnectable, 9-port or 17-port star couplers may be constructedaccording to the invention. In this case, 8 and 16 branching equalbranching circuit units are used.

FIG. 28 is a diagram showing an interconnectable star coupler with apair of 5-port groups according to a second embodiment of the invention.As shown, optical waveguides, a 1×2 equal branching circuit 205, and a2×2 equal branching circuit 206 are formed on the substrate 201. Opticalfibers 202 are derived from the substrate 1, corresponding to therespective ports.

In the second embodiment of the FIG. 28, the 1×2 equal branching circuit205 is used in place of the 1×2 fiber coupler 231, and the 2×2 equalbranching circuit 206 is used in place of the 2×2 fiber coupler 232. Ina conventional fiber coupler, only the Evanescent optical coupler can bemanufactured with some restrictions on the manufacturing. On the otherhand, use of the equal branching circuit 205 is allowed in theintegrated optical circuit of the second embodiment shown in FIG. 28. Inthe fiber coupler, it is difficult to directly couple two optical fibersshaped circular in cross section. For this reason, two optical fibersshaped circular in cross section are located closely, and it is filledwith medium of low refractive index in a manner that the mediumsurrounds the optical fibers. Therefore, only the optical coupler by theEvanescent wave coupling can be formed. On the other hand, in theintegrated optical circuits, optical waveguides are formed in or on thesubstrate by the photolithographic technique. Accordingly, it is veryeasy to manufacture two optical waveguides directly coupled, therebyeliminating the junction loss. Further, in the instant embodiment, theangle δ at the intersecting portions in the integrated optical circuitis larger than the critical angle θ of the optical waveguide. Thiseliminates the interference between the optical waveguides.

The 2×2 equal branching circuit 206 may be either of the junction typeas shown in FIG. 26(c) or the Evanescent optical coupler type. In thiscase, the Y equal branching circuits 205 must be provided at the portsconnecting to the optical fibers 202. In FIGS. 26(a) and 26(c), theoptical waveguides 231 and 232 or 233 and 234 intersect at an angle,which is much smaller than the critical angle θ.

In the second embodiment of FIG. 28, the light signal is merelydistributed to the port groups oppositely disposed. Accordingly, opticalwaveguides, which are less bent than those of the prior art or the FIG.25 embodiment, may be used. Accordingly, the loss caused by the bendingof the optical waveguide can be reduced. The loss caused by the bendingof the optical waveguide of the single mode cannot be described ingeometrical optics. The fact that the loss increases with decrease ofthe curvature is empirically confirmed. The loss causing mechanism iscomplicated, involving two factors, radiation loss and modetransformation loss. (For more details, reference is made to Azuma andKuwaki "A study on the loss change mechanism at an optical fiber bendingregion", The 1992 IEICE spring conference, B-893 (1992)).

FIG. 29 shows an interconnectable star coupler with a pair of 9-portgroups according to a third embodiment of the invention. The thirdembodiment is different from the second embodiment in that the number ofports is increased from five to nine. The increase of the number ofports makes the integrated optical circuit complicated. Accordingly,intersecting portions 207 where three optical waveguides intersect arepresent, as shown in FIG. 29. Also in this case, angles α, β and γformed by the waveguides 221, 222, and 223, shown in FIG. 30(a), areselected to be larger than a value two times of the critical angle θ ofthe optical waveguide. When the number of optical waveguides isincreased, scattered light will increase. To avoid this, it issuggestible to slightly change the paths of the waveguides so as not tocause the intersecting portions of three or more waveguides.

According to the invention, the single mode, interconnectable multi-portstart coupler can be constructed not using the 1×2 Evanescent opticalcoupler. Therefore, the star coupler is free from the junction losscaused by the 1×2 Evanescent optical coupler. The circuit can beconstructed by using merely the combination of the 1×2 equal branchingcircuit and the 2×2 equal branching circuit. Therefore, also in themulti-mode, interconnectable star coupler,. its manufacturing is easy.

Further, embodiments of an optical coupler will be described withreference to a single mode optical waveguide. FIG. 31 is a plan viewshowing an embodiment of an optical coupler according to the presentinvention. The optical coupler shown in FIG. 31 is a optical couplerformed by connecting in series an asymmetrical Y branching circuit 302and an Evanescent optical coupler 304. The asymmetrical Y branchingcircuit involves the branching circuit of the type in which two opticalwaveguides having different shapes in cross section are branched fromthe original optical waveguide at equal angles with respect to thelatter, the branching circuit of the type in which two opticalwaveguides having the same shapes in cross section are branched from theoriginal optical waveguide at different angles with respect to thelatter, and the branching circuit of the type in which two opticalwaveguides having different shapes in cross section are branched fromthe original optical waveguide at different angles with respect to thelatter. In the asymmetrical Y branching circuit 302, an opticalwaveguide 302b rectilinearly extends in align with the optical waveguideof the common part 301. The cross sectional area of the opticalwaveguide 302b is larger than that of an optical waveguide 302a. In theEvanescent optical coupler 304, two optical waveguides 304a and 304b areequal. Accordingly, in a junction part 303, the cross sectional area ofthe waveguide 303a increases toward the right side. At the interface ofthe junction part 330 and the Evanescent optical coupler 304, theoptical path difference is adjusted so that the two waveguides are inphase, viz., the phase matching condition to be described later issatisfied.

In the single mode optical waveguide, as shown in FIG. 32, most of lightfrom the common part 301 goes to the waveguide 302b, and the light goeslittle or no to the waveguide 302a. This property of single modeasynmetric y-branching circuit is described in: Bures et al. "ModeConversion In Planar-Dielectric Separating Waveguide", IEEE J. QauntumElectron., vol. QE-11, No. 1, pp32-39 (1975). In the Evanescent opticalcoupler 304, light propagating through the waveguide 304b is branched tothe waveguide 304a by the mode coupling. Accordingly, the opticalcoupler of FIG. 31 functions as an unequal optical coupler.

As shown in FIG. 33, a light signal coming in through an opticalwaveguide 305b of the branching portion 305 propagates into thewaveguide 304b of the Evanescent optical coupler 304. In this coupler,the light signal is branched into the waveguide 304a by the modecoupling. The light signal from the waveguide 4b and the light signalbranched into the waveguide 304a are joined together in the asymmetricalY branching circuit 302. At this time, if both the light signals are outof phase, one of the light signals negates the other, resulting insignal loss. However, in this embodiment, the optical path difference isadjusted so as to satisfy the phase matching condition. Accordingly,this loss is not created.

As shown in FIG. 34, a light signal coming in through an opticalwaveguide 305a of the branching portion 305 propagates into thewaveguide 304a of the Evanescent optical coupler 304. In this coupler,the light signal is branched into the waveguide 304b by the modecoupling. The light signal from the waveguide 304a and the light signalbranched into the waveguide 304b are joined together in the asymmetricalY branching circuit 302. Also in this case, the optical path differenceis adjusted so as to satisfy the phase matching condition. Accordingly,this loss is not created.

FIG. 35 is a diagram showing a model of optical paths in the opticalcoupler of FIG. 31. In the asymmetrical Y branching circuit 32, theoptical path difference of δ=γ sing θ is caused. γ indicates theinterval of the optical waveguides in the Evanescent optical coupler304, and θ, an branching angle of the asymmetrical Y branching circuit302. When the optical path difference δ is a multiple of the wavelengthof propagating light, the phase matching condition is satisfied.Accordingly, the phase matching condition depends on the wavelength ofthe propagating light. The optical coupler of FIG. 31 must be designedaccording to the wavelength used.

FIGS. 36 and 37 show other embodiments as modifications of the FIG. 31optical coupler. In those embodiments, the optical path difference δ ofthe waveguides are set to zero (0). Therefore, the phase matchingcondition does not depend on the wavelength.

The embodiment of FIG. 36 includes an asymmetrical Y branching circuit36 in which optical waveguides having different sectional areas aresubstantially symmetrically branched at angles ω1 and ω2. The sectionalarea of the waveguide 306a is smaller than that of the waveguide 306b inFIG. 36. Further, the waveguide is branched at an angle ω1-ω2 withrespect to the original waveguide 301. The waveguide of the smallersectional area has a smaller refractive index. Accordingly, the lightpropagating speed is larger. If ω1=ω2, the phase matching condition isnot satisfied. For this reason, ω1 is set to be slightly larger than ω2so that the optical path difference δ is 0.

In the FIG. 37 embodiment, an asymmetrical Y branching circuit 307 andan optical path difference adjusting part 308 are provided so that theoptical path difference δ is 0. In the asymmetrical Y branching circuit307, optical waveguides 307a and 307b having equal sectional areas arebranched at different angles with respect to the original opticalwaveguide 301. The waveguide 307a rectilinearly extending in theasymmetrical Y branching circuit 307 is bent in its optical path in theoptical path difference adjusting part 308. As a result, in theEvanescent optical coupler 304, the optical path difference δ is 0.

As known, the coupling characteristic of the Evanescent optical couplerdepends on the wavelength. This must be allowed for when designingactual optical couplers.

As seen from the foregoing description, the optical couplers of FIGS.31, 36, and 37 function as optical couplers free from the junction loss.

FIG. 38 is a plan view showing an embodiment of an interconnectable starcoupler according to the invention. In this star coupler, Evanescentoptical couplers in a conventional star coupler are substituted by theoptical couplers 11a to 11d shown in FIG. 31, 36 or 37. Use of theoptical couplers 315a to 315d eliminates the junction loss. Accordingly,the star coupler of this embodiment is interconnectable and has lessloss.

FIG. 39 is an embodiment as a modification of the FIG. 38 star coupler.In the star coupler of FIG. 39, a reflecting means 325 is provided inthe optical waveguide circuit. The waveguide is bent substantially atright angle to form a bent portion 314. Accordingly, no circularwaveguides as found in FIG. 38 are used. This reduces the area of thesubstrate required for the integrated optical circuit formation,realizing the size reduction of the star coupler. In the construction ofFIG. 38, the branches of small branching ratio of the optical couplers311a to 311d are connected to opposed terminals, while in theconstruction of FIG. 39, the branches of small branching ratio areconnected to adjacent terminals. In the construction of FIG. 39, an Xbranching circuit 313 is used instead of the Evanescent optical coupler312 in FIG. 38. If required, it may be the Evanescent optical coupler312.

An enlarge view of the Evanescent optical coupler 312 is shown in FIG.40(a). An enlarged view of the bent portion 314 is shown in FIG. 40(c).

In FIG. 40(a), a light signal coming in through the optical waveguide321a is equally distributed into optical waveguides 322a and 322b,through the mode coupling action in a coupling part 324 where twooptical waveguides are closely arrayed side by side. A light signalcoming in through another optical waveguide 321b is also equallydistributed into optical waveguides 322a and 322b in a similar way.

In FIG. 40(b), a light signal coming in through the optical waveguide321a is equally distributed into optical waveguides 322a and 322b,through a mixing optical waveguide 324. A light signal coming in throughanother optical waveguide 321b is also equally distributed into opticalwaveguides 322a and 322b in a similar way.

In FIG. 40(c), a light signal coming in through the optical waveguide321a is reflected by the reflecting means 325 to be bent toward theoptical waveguide 322. The reflecting means 325 is a total reflectionmirror fabricated on the integrated optical circuit by dry etchingprocess, in this instance of the embodiment. Such a technique is known(see Shibata, Okuda, Ikeda, and Monda, "Branching characteristic ofmulti-stage connected asymetric y-branch using total reflection", The1992 IEICE (institute of electronics/information/communicationengineering) Spring Conference, C-198 (1992)).

The approach using the reflecting means in the optical waveguide morecontributes to the size reduction of the integrated optical circuit thanthe conventional star coupler.

The present invention is also applicable for the 1×3 optical coupler. Aninterconnectable star coupler with four ports constructed using the 1×3optical coupler 315 of the invention is illustrated in FIG. 41. Thestructure of a 1×3 branching circuit 317 is basically asymmetrical.Accordingly, if it is connected in series with an Evanescent opticalcoupler 318 (three optical waveguides are arrayed in parallel), the 1×3optical coupler 315 free from the junction loss can be formed.Accordingly, an interconnectable star coupler with four ports can beformed by combining four number of such 1×3 optical couplers 315.Because of the rectilinear propagation of light, light signalspropagating through the waveguides crossing at right angles do notinterfere with each other in the intersecting portion 316 of thewaveguides.

The optical coupler and the interconnetable star coupler of theinvention can be fabricated by a know optical waveguide (integratedoptical circuit) fabricating method, such as an ion exchange method or aflame deposition method.

As described above, the optical coupler with less junction loss can berealized. When the optical coupler is incorporated into the integratedoptical circuit, an interconnectable star coupler with less loss can berealized. Provision of the reflecting means in the circuit allows theoptical waveguide to be bent at substantially right angle. This leads tosize reduction of the integrated optical circuit containing theinterconnectable star coupler.

What is claimed is:
 1. An interconnectable star coupler in an opticalcommunication network having N (N=2^(i) +1, i is an integer ≧2)terminals, comprising:a substrate: an equal branching circuit unitincluding a plurality of 1×2 equal branching circuits corresponding tosaid terminals, which are arranged on the substrate; said equalbranching circuit unit being connected through optical waveguides to thecorresponding terminals in such a manner that there are intersectingportions of said optical waveguides in said star coupler.
 2. Theinterconnectable star coupler according to claim 1 wherein an anglebetween said waveguides exceeds an angle twice as much as a criticalangle of said waveguides.
 3. An interconnectable star coupler in anoptical communication network having N (N=2^(i) +1, i is an integer ≧2)pairs of terminals, comprising:N pairs of 1×2 equal branching circuitscorresponding to N pairs of terminals; and at least one 2×2 branchingcircuit coupled between said terminals and said 1×2 equal branchingcircuits, branch terminals of at least one of said 1×2 equal branchingcircuits being connected through optical wave guides to correspondingterminals of said at least one 2×2 equal branching circuit in such amanner that said optical waveguides in said star coupler intersect. 4.The interconnectable star coupler according to claim 3 wherein an anglebetween said waveguides exceeds an angle twice as much as a criticalangle of said waveguides.
 5. An interconnectable star coupler accordingto claim 1 wherein said each branching circuits comprise an opticalcoupler comprising a Y branching circuit connected in series with anEvanescent optical coupler.
 6. The optical coupler according to claim 5wherein said Y branching circuit is an asymmetrical Y branching circuit.7. The optical coupler according to claim 5 wherein an optical pathlength is adjusted to satisfy a phase matching condition at a mixingportion of said Y branching circuit.
 8. The optical coupler according toclaim 7 wherein an optical path difference between said opticalwaveguides is eliminated to adjust the phase matching condition.
 9. Aninterconnectable star coupler comprising:an optical coupler including:aY branching circuit; two optical waveguides having a first refractiveindex and of approximately equal cross-sectional shapes, each opticalwaveguide coupled to a different branch of the Y branching circuit; anda medium, with a second refractive index, lower than the firstrefractive index, between the optical waveguides, whereby energy istransferred from one waveguide to the other waveguide.
 10. Theinterconnectable star coupler according to claim 9 wherein said Ybranching circuit includes branches of different cross-sectional shapes.11. The interconnectable star coupler according to claim 9 wherein anoptical path length is adjusted, based on a distance between the opticalwaveguides, to satisfy a phase matching condition at a mixing portion ofsaid Y branching circuit.
 12. The interconnectable star coupleraccording to claim 9, wherein the branches of the Y branching circuitare arranged at substantially symmetrical angles, a smallercross-sectional branch having a lower refractive index than a largercross-sectional branch, and wherein an optical path difference betweensaid optical waveguides is eliminated to adjust a phase matchingcondition.